Engineering plastic is a term generally referring to plastic having a lower weight and higher strength, and particular examples thereof include polyimide, polycarbonate, or the like. Polybenzimidazole is a material having the highest thermal strength among such engineering plastics and recently have been used widely as a material for fire-fighting garments, space suits, or the like (Non-patent Document 1).
However, according to the conventional polymerization process for polybenzimidazole, a strongly acidic polyphosphoric acid solvent is used both as a polymerization solvent and as a dehydration reagent, and acidic isophthalic acid is also used as a monomer. Thus, a seriously complicated post-treatment process is required for neutralizing such acidic materials, thereby limiting industrial mass production of polybenzimidazole (Non-patent Document 2).
In addition, a polymerization method for polybenzimidazole using phenylsulfone or sulfolane as a solvent is known. However, the reaction condition has not yet been optimized. Moreover, such a polymerization method is applicable merely at a significantly diluted concentration and thus is not suitable for mass production (Non-patent Document 3).
Meanwhile, many studies and results thereof have been disclosed with reference to polybenzimidazole. However, most of them are limited to introduction of a filler to conventional polybenzimidazole or preparation of a copolymer. Thus, there has been no special study about the polymerization method for polybenzimidazole itself other than the above-mentioned methods.
Under these circumstances, the present inventors have recognized that substitution of such a strongly acidic solvent and monomer used for the conventional polymerization process for polybenzimidazole with another solvent and monomer may facilitate mass production of polybenzimidazole for engineering plastic. Therefore, there has been suggested a novel method for preparing an isophthalaldehyde bisulfite adduct, which is a monomer for use in polymerization of polybenzimidazole under a relatively mild condition in an organic solvent.
Particularly, one of the most important factors in preparing an isophthalaldehyde bisulfite adduct as a monomer is purity. According to the Carothers equation as depicted by the following Formula 1, it can be seen that even a slight decrease in conversion (p, 0≤p≤1) causes a significant decrease in degree of polymerization (DP, Xn).
                              [                      Formula            ⁢                                                  ⁢            1                    ]                ⁢                                                                                                            X            n                    _                =                  1                      1            -            p                                              (                  Carothers          ⁢                                          ⁢          equation                )            
Therefore, when the monomer does not have high purity, the conversion is decreased inevitably. Thus, there is a disadvantage in that it is not possible to obtain polybenzimidazole having a high molecular weight.